The present invention relates to a process for preparing .alpha.-olefin oligomers. More particularly, it relates to an industrially advantageous process of formation of .alpha.-olefin oligomers in which it is possible to produce .alpha.-olefin oligomers principally composed of 1-hexene from ethylene with a high selectivity and in a high yield.
As an oligomerization process of .alpha.-olefins such as ethylene, etc., methods are known in which a chromium-based catalyst composed of a specific chromium compound and a specific organoaluminum compound is used as a catalyst. For instance, Japanese Patent Publication (KOKOKU) No. 43-18707 discloses a process for producing 1-hexene from ethylene by using a catalyst consisting of a chromium-containing VIB Group transition metal compound and a polyhydrocarbylaluminum oxide.
Also, in Japanese Patent Application Laid-Open (KOKAI) No. 3-128904 is disclosed a method for oligomerizing .alpha.-olefins by using a catalyst obtained by reacting a chromium-containing compound having chromium-pyrrolyl bond with an alkyl metal or a Lewis acid. South Africa Patent Publication ZA93/0350 describes a process for producing an .alpha.-olefin oligomer using a catalyst system obtained by mixing a chromium compound, a pyrrole-containing compound, a metal alkyl compound and a halide source in a mutual solvent.
However, according to the process of Japanese KOKOKU No. 43-18707, the amount of the by-product polyethylene which is produced with 1-hexene is large, and if such reaction is carried out under the conditions which decrease yield of the by-product polyethylene, the catalyst activity is lowered.
Also, the method of Japanese KOKAI No. 3-128904 has the problem that the activity of the catalyst is unsatisfactory, although the yield of the by-products such as polyethylene, etc. is low. Further, in this method, there are required, in addition to the .alpha.-olefin oligomerization process, a step for preparing a chromium-containing compound at least having chromium-pyrrolyl bond from a chromium salt and metal pyrrolide, and a step for isolating the said chromium-containing compound, so that not only the oligomerization operations become too complicated but also the construction cost for the whole process is elevated. Moreover, it is very difficult to handle the chromium-containing compounds at least having chromium-pyrrolyl bond, since they are very sensitive to air and to moisture.
South Africa Patent ZA93/0350 discloses that when an unsaturated hydrocarbon for improving a stability of the catalyst system is present in excess, the selectivity and/or activity of the catalyst are lowered. Also, the process described in South Africa Patent ZA93/0350 is defective in that since it also requires a preliminary preparation process of an unstable catalyst and an isolation process of the catalyst, the operation is not only complicated but also requires a high construction cost for the entire process.
As a result of the present inventors' earnest studies in order to solve the above-mentioned problems, it has been found that by carrying out oligomerization of an .alpha.-olefin in a reaction solvent by reacting the .alpha.-olefin in a catalytically effective amount of a chromium-based catalyst system comprising a combination of at least a chromium compound, a nitrogen-containing compound of at least one selected from the group consisting of an amine, an amide and an imide, an alkylaluminum compound and a halogen-containing compound, in a contacting mode that the chromium compound is not contacted with the alkylaluminum compound each other before each of the chromium compound, the nitrogen-containing compound of at least one selected from the group consisting of an amine, an amide and an imide, the alkylaluminum compound and the halogen-containing compound and .alpha.-olefin coexist in the reaction solvent, and the oligomerization of .alpha.-olefin takes place, .alpha.-olefin oligomers are obtained in a high yield. On the basis of this finding, the present invention has been attained.